Hydrostatic transmissions have many and varied uses, including the propelling of vehicles, such as mowing machines, and offer stepless control of such a machine's speed. A typical hydrostatic transmission system includes a variable displacement main pump connected, in a closed hydraulic circuit, with a fixed displacement hydraulic motor. In order to avoid possible cavitation or excessive pressure of the closed loop, a pair of auxiliary valves is required, with each such valve consisting of a check valve and a pressure relief valve. Thus, this auxiliary or combination valve acts as a combined pressure relief and anti-cavitation valve and is sometimes also referred to as a port relief valve. Typically, such a valve is manufactured from high strength steel and basically consists of a valve seat, a valve stem, a relief valve spring, a spring cap a check valve spring and, upon installation, of an optional valve plug. The combination valve can utilize different relief pressure settings, depending upon the size of the relief valve opening and the preload of the relief valve spring. Adjustment of the working height of the relief valve spring will change the pressure setting during the initial valve assembly. Thus, after the pressure setting has been accomplished, the spring cap is locked, such as via crimping or welding, onto the valve stem so as to bias the intermediate relief valve compression spring against the valve seat.
Some prior art designs that utilize crimping to lock the spring cap to the valve stem do not utilize a positive engagement mechanism between the spring cap and the valve stem prior to the noted crimping process. As a result, the spring cap can axially move, thus making the fine adjustment of the relief valve pressure setting difficult to achieve. The crimping process of the valve cap to the valve stem has to be carried out prior to the testing of the valve and if the valve, after test, does not meet the expected pressure setting, it is discarded, resulting in unnecessary cost and waste.
The patent literature, which encompasses a large number of the previously described types of auxiliary or combination valve structures, includes, among others: U.S. Pat. No. 3,112,763 to Tennis, et al.; U.S. Pat. No. 4,210,170 to Sutton; U.S. Pat. No. 4,300,591 to Sutton; U.S. Pat. No. 5,139,047 to Reinicke; U.S. Pat. No. 6,056,263 to Stier; U.S. Pat. No. 6,691,512 B1 to Kopel et al.; U.S. Pat. No. 6,719,005 B1 to Trimble et al.; U.S. Pat. No. 6,761,182 B1 to Trimble et al; and UK Patent Application GB 2 064 069 A to Kotter et al. However, none of these prior art structures pertain to the use of a spring cap that utilizes four successively adjoining axial portions, each of which has a particular specific function, in combination with an axially, centrally apertured, valve seat that also utilizes a first plurality of axial cross-port orifice holes, all having substantially the same diameter, and a second plurality of circumferentially-spaced, radial, semicircular holes emanating from the valve seat central aperture, wherein the first and second pluralities of holes are perpendicular to and intersect each other.